30.08.2024 17:50, Andrew Jones пишет:
> On Fri, Aug 30, 2024 at 02:05:05PM GMT, Vladimir Isaev wrote:
>> Hi Andrew,
>>
>> 29.08.2024 11:40, Andrew Jones wrote:
>>> While the spec doesn't state it, setting timecmp to UINT64_MAX is
>>> another way to stop a timer, as it's considered setting the next
>>> timer event to occur at infinity.
>>
>> I think this should be explicitly stated in the spec, since some software
>> may initially set time and timecmp to big values just to check how overflow
>> is handled. And without it no chance that all HW implementations will
>> interpret
>> UINT64_MAX as 'stop timer'.
>>
>> Do we need github issue on SSTC/privileged?
>
> Hi Vladimir,
>
> I don't think we need to update the spec, so hardware is free to make the
> interrupt pending if time ever reaches UINT64_MAX. However, the hardware
> will then need to clear the pending interrupt on the very next increment
> of time. This means even if the interrupt is raised it will most likely
> look spurious to the handler. I think we can leave this to software. If
> the software wants to use UINT64_MAX to stop timers, then it may also want
> to add a check in its timer handlers for timecmp == UINT64_MAX, and, under
> that condition, just ignore the interrupt.
>
oh, I got it, thank you for explanation!
> Thanks,
> drew
>
>>
>> Thank you,
>> Vladimir Isaev
>>
>>> And, even if the time CSR does
>>> eventually reach UINT64_MAX, the very next tick will bring it back to
>>> zero, once again less than timecmp. For this reason
>>> riscv_timer_write_timecmp() special cases UINT64_MAX. However, if a
>>> previously set timecmp has not yet expired, then setting timecmp to
>>> UINT64_MAX to disable / stop it would not work, as the special case
>>> left the previous QEMU timer active, which would then still deliver
>>> an interrupt at that previous timecmp time. Ensure the stopped timer
>>> will not still deliver an interrupt by also deleting the QEMU timer
>>> in the UINT64_MAX special case.
>>>
>>> Fixes: ae0edf2188b3 ("target/riscv: No need to re-start QEMU timer when
>>> timecmp == UINT64_MAX")
>>> Signed-off-by: Andrew Jones <ajo...@ventanamicro.com>
>>> ---
>>> target/riscv/time_helper.c | 1 +
>>> 1 file changed, 1 insertion(+)
>>>
>>> diff --git a/target/riscv/time_helper.c b/target/riscv/time_helper.c
>>> index 8d245bed3ae3..bc0d9a0c4c35 100644
>>> --- a/target/riscv/time_helper.c
>>> +++ b/target/riscv/time_helper.c
>>> @@ -92,6 +92,7 @@ void riscv_timer_write_timecmp(CPURISCVState *env,
>>> QEMUTimer *timer,
>>> * equals UINT64_MAX.
>>> */
>>> if (timecmp == UINT64_MAX) {
>>> + timer_del(timer);
>>> return;
>>> }
>>>
>>
>>
>>
